I was wondering is there a way to know if a video has been edited or not? Like say someone take a video using their webcam or whatever device and send me and i want to be sure that it is not "fake", edited?
What you are touching on is the field of *image forensics**. There is no simple way to determine whether or not a video has been edited (if it has been well done), but there are many subtle things that might give you a hint. There are both technical and visual giveaways. Technical giveaways are artifacts of the editing technique, for example, editing software use known mathematical formula to produce effects like blurring that do not match reality, so by closely examining a blurred edge (often with the help of mathematical analysis) you might be able to determine whether it is a natural blur or one created by an editing program.
You might know that this technique is also used on still images - still image forensics and motion image forensics use many of the same techniques, but they are more complicated when applied to video. Of course, there are also extra techniques that can be used on video, since you have motion to deal with ('faked' videos often seem to have fake-looking motion because of the difficulty of matching motion precisely and creating good random motion).
Visual analysis comes down to simply looking very closely to see that everything is consistent. For example, differences between a shadow and the object casting it, or the angle of a shadow through the video, might indicate that something has been changed. Of course, a competent video editor will be careful to insert proper shadows. You might have to look at much smaller clues.
Forensic image analysis is something that people must study and practice extensively in order to gain competence, and even for a highly skilled analyst it can be difficult to make an absolute determination, simply because there are also highly skilled video editors.
I was curious about the career path for image forensics, so I looked in to the title of such analysts a bit. Forensic Document Examiners, which are well established in courts, do not generally deal with photographs (FDEs, instead, specialize in things like typography, handwriting, signatures, etc). It looks like most image analysts have a background in Computer Science or Information Assurance, which makes sense because many analysis techniques have to do with the details of image compression algorithms. Some of the researchers in the area are mathematicians.
Here's a few articles I found that list some common techniques:
*Terminology note: image forensics seems to refer to the practice of analyzing an image or video to determine how (or if) it has been altered. video forensics generally refers to the practice of analyzing a video that is assumed to be unalterede in order to extract information from it (for example, enhancing a frame from surveillance footage to produce a clear image of a suspect). So, what you are asking about is image forensics on a video. This conflict of terminology is confusing and annoying.
By "video", presumably you mean a digital video file. By edit, presumably you mean that frames or scenes have been cut. By "edit" I assume you are not referring to image manipulation, like inserting a person into the scene, which could be detected by incorrect lighting or shadows etc.
Consider the simpler case of a text file. If you're provided a text file, could you tell if it has been "edited"? Probably not, unless you have the original file or a copy of it, with known and good provenance.
A digital video file is a data file just like a text file. You probably would not be able to tell if it has been edited if someone wants to deceive you.
Finding out if a file is altered after the fact is hard. However, when you create videos and want to make sure they arent altered, you can do a MD5 hash on the file. At a later date if you run the hash again on the file and it comes up different you know it has been altered.
To officially prove a video has been doctored is very difficult if the perpetrators do a good job, however if you personally analyze a video file that has indeed been doctored with careful examination you will notice anomalies and know the truth for yourself. Like anything else in life if you put enough effort in learning the craft/technique you will become good at it.
all OSs can show last modification date when giving a directory listing.
AFAIK, the best method would be to put a transparent text or image overlay into the video. I don't think there is an easier way to provide digital signage to a video. . .
I'm, of course, assuming MPEG format.